Measurement or confirmation of feature size or “critical dimension” (“CD”) confirms accuracy and tolerances at many stages of microelectronic device manufacture. For example, CD may be measured upon production of a partially transmissive mask before utilizing said mask to produce layered devices. Subsequent to utilizing a mask to pattern one or more layers of a microelectronic device, CD may again be measured to confirm geometric accuracy and tolerances.
As device sizes continue to shrink, accurately measuring the size of features becomes increasingly difficult. Current methods to provide accurate and precise measurements of CD of small features inexpensively are unsatisfactory. For example, current techniques such as those utilizing scanning electron microscopy or optical metrology may lack the requisite precision due to substrate charging, or may be limited physically by the Raleigh resolution limit associated with light microscopy. In measuring CD of a mask, both current optical and SEM metrology tools lack the ability to measure CD smaller than about 250 nm, due to diffraction and substrate charging. In measuring CD of features on a wafer, current tools lack the ability to measure CD smaller than about 90 nm.